Better Rendering with focus one position?
I am not sure why you would want this, as the end result will always be the same, but if it is just that you can see one part of the shot fully rendered than have a look at this:


Tiles
Under Performance you choose your tile-size which is the size of one block (in pixels). You can set it to start from whichever part of the image you like (where it says centre), so if you have something in your shot which you want to see first rendered to the fullest make sure you set it to start, wherever the item of interest is.

Threads
You can also set a fixed number of threads and put it to one this way you will always have one tile processing at a time (but that just doesn't use the full capacity of your machine, so i suggest leave it at auto).

Progressive Refinement
What, I assume, you don't want is the progressive refinement box ticked, that will cause it to use another algorithm, which will show you the whole picture very quickly, and as the name suggests will refine it progressively over time. So make sure this is un-ticked, for achieving what you want.